370 INTERNATIONAL ASTRONOMICAL UNION Proc. N. A. S. 
a period of some twenty years, a review of the results at this time is of 
interest : 
Linear Vei.ocity of Solar Rotation at the Equator 
OBSERVER 
VELOCITY 
NO. OF LINES 
REGION 
DATE 
2.08 
2 
6301-6302 
1900-5 
Halm 
2.04 
2 
6301-6302 
1904 
Adams 
2.06 
20 
4196-4294 
1907 
2.05 
22 
4196-4291 
1908.5 
2.08 
10 
6280-6318 
1909 
Plaskett, J. S 
2.01 
19 
5506-5688 
1911 
Plaskett, J. S 
2.02 
15 
4196-4291 
1911 
DeLury 
1.97 
19 
5506-5688 
1911 
Hubrecht 
1.86 
40 
4299-4400 
1911 
Plaskett, J. S 
2.01 
27 
4250-5600 
1911-12-13 
2.00 
20 
4058-4276 
1912 
Evershed and Royds 
1.95 
3906-5624 
1913 
Plaskett, H. H 
1.98 
12 
5574-5628 
1913 
1.94 
35 
4123-4338 
1914 
Plaskett, H. H 
1.95 
5 
5900 
1915 
St. John and Ware 
1.94 
26 
5018-5316 
1914-18 
St. John and Ware 
1.95 
7 
6265-6337 
1916-17 
In view of the accuracy attainable in spectro-photographic measures the 
differences between these results are surprising and disappointing and are 
the outstanding feature in the problem of solar rotation. They are larger 
than the probable errors of measurement, and either depend upon sys- 
tematic errors of the measurers or they are of solar or terrestrial origin. 
The tracing of them to their source is the line along which investigation 
may be directed with great advantage. 
Investigations now in progress at Mount Wilson, in which simultaneous 
observations are made upon the two limbs and the center of the sun, 
indicate that temporary and local conditions in the sun's reversing layer are 
frequently such as to produce differences of ten per cent in the rotation 
values obtained by comparing east and west limbs directly. 
The observations show further that high values at one limb are not 
correlated with high values at the other as they would be if they were due 
to real changes in the rotation of the reversing layer or to conditions in 
the terrestrial atmosphere. In the light of these results it is clear that a 
short series of observations may give misleading results and it even be- 
comes a question whether the solar rotation can be determined as defi- 
nitely as has been thought. The means of extended series of observations 
taken under constant conditions should, however, give results of great 
value. Such a series at Mount Wilson Observatory is now in its sixth 
year. The observations show no definite evidence of periodicity in the 
sun's rotation from 1914 to 1919, but furnish strong indications of fre- 
